The interface of a zinc-terminated polar zinc oxide surface (0002) with a series of chemisorbed fluorinated benzylphosphonic acids has been studied using density functional theory. The calculations indicate that there is a substantial change in the binding energies and work function modification depending on the binding motif. The results also indicate that there is a pronounced difference in the magnitude and trends of the factors determining the total change in work function. The oxygen core-level binding shifts have been calculated and compared to available experimental data.

No description available.

<p>Molecular electronics is envisioned as a possible next step in device miniaturization. It is usually taken to mean the design and manufacturing of electronic devices and applications where organic molecules work as the fundamental functioning unit. It involves the easurement and manipulation of electronic response and transport in molecules attached to conducting leads. Organic molecules have the advantages over conventional solid state electronics of inherent small sizes, endless chemical diversity and ambient temperature low cost manufacturing.</p><p> In this thesis we investigate the switching and conducting properties of photochromic dithienylethene derivatives. Such molecules change their conformation in solution when acted upon by light. Photochromic molecules are attractive candidates for use in molecular electronics because of the switching between different states with different conducting properties. The possibility of optically controlling the conductance of the molecule attached to leads may lead to new device implementations.</p><p> The switching reaction is investigated with potential energy calculations for different values of the reaction coordinate between the closed and the open isomer. The electronic and atomic structure calculations are performed with density functional theory (DFT). It is concluded that there is a large potential energy barrier separating the open and closed isomer and that switching between open and closed forms must involve excited states. </p><p>The conducting properties of the molecule inserted between gold leads is calculated within the Non Equilibrium Green Function theory. The transmission function is calculated for the two isomers with different basis sizes for the gold contacts, as well as the electrostatic potential, for finite applied bias voltages. We conclude that a Au 6s basis give qualitatively the same result as a Au spd basis close to the Fermi level. The transmission coefficient at the Fermi energy is around 10 times larger in the closed molecule compared to the open. This will result in a large difference in conductivity. It is also found that the large difference in conductivity will remain for small applied bias voltages. The results are consistent with earlier work.</p>

<p>The purpose of this thesis has been to investigate the elastic properties of an fcc FeMg alloy with 10 at.% magnesium under high pressure. Recent research has shown that magnesium can be a possible candidate for light element impurities in the Earth’s inner core, something that was previously not considered possible because of the low miscibility of magnesium in iron at ambient pressure. Gaining knowledge about the composition of the Earth’s core can help us better understand such phenomena as seismic activity and the fluctuations of the Earth’s magnetic field.</p><p>The elastic constants of the FeMg alloy was calculated using ab-initio methods based on Density Functional Theory. The Exact Muffin-Tin Orbitals method was used in conjunction with the Coherent Potential Approximation.</p><p>The FeMg alloy was found to be overall considerably softer than pure iron, and the softening effect on the elastic constants was also found to increase with pressure. The results also showed that 10% Mg alloying increased the anisotropy with about 40% compared to pure iron.</p>

As an important chemical raw material, alcohols can be used as fuels, solvents and chemical feedstocks to produce a variety of downstream products. With limited fossil fuel resources, alcohol synthesis from syngas reactions can be a potential alternative to the traditional petroleum based alcohol synthesis. Among many catalysts active for syngas to alcohol processes, alkali promoted Mo2C has shown promising performance. More interestingly, the alkali promoter was found to play an important role in shifting the reaction selectivity from hydrocarbons to alcohols. However, limited understanding of the mechanism of this alkali promoter effect is available due to the complexity of syngas reaction mechanism and low content of alkali added to the catalysts. In this thesis, we performed a comprehensive investigation of the alkali promoter effect with density functional theory (DFT) calculations as our primary tool. We first examine various Mo2C surfaces to determine a representative surface structure active to alkali adsorption. On this particular surface, we develop a syngas reaction network including relevant reaction mechanisms proposed in previous literature. With energetics derived from DFT calculations and a BEP relation, we predict the syngas reaction selectivity and find it to be in excellent agreement with experimental results. The dominant reaction mechanism and selectivity determining steps are determined from sensitivity analysis. We also propose a formation mechanism of alkali promoters on Mo2C catalysts that shows consistency between experimental IR and DFT computed vibrational frequencies. Finally, the effect of alkali promoters on the selectivity determining steps for syngas reactions are investigated from DFT calculations and charge analysis. We are able to rationalize the role of alkali promoters in shifting the reaction selectivity from hydrocarbons to alcohols on Mo2C catalysts.

CO hydrogenation and CO oxidation are two important processes addressing the energy and environmental issues of great interest. Both processes are carried out using metallic catalysts. The objective of this dissertation is to study the catalytic processes that govern these two reactions from a molecular perspective using quantum mechanical calculations. Density Functional Theory (DFT) has proven to be a valuable tool to study adsorption, dissociation, chain growth, reaction pathways etc., on well-defined surfaces. DFT was used to study the CO reduction reactions on promoted cobalt catalyst surfaces and CO oxidation mechanisms on cobalt surfaces. CO hydrogenation via Fischer-Tropsch Synthesis (FTS) is a process used to produce liquid fuels from synthesis gas. The economics of the Fischer-Tropsch process strongly depends on the performance of the catalyst used. The desired properties of a catalyst include selectivity towards middle distillate products such as diesel and jet fuel, higher activity and longer catalyst life. Catalysts are often modified by adding promoters to obtain these desirable properties. Promoters can influence the reaction pathways, reducibility, dispersion, activity and selectivity. In FTS, understanding the effect of promoters in the molecular scale would help in tailoring catalysts with higher activity and desired selectivity. Preventing deactivation of catalyst is important in FTS to increase the catalyst life. Deactivation of Co catalyst can occur by reoxidation, C deposition, sintering, formation of cobalt-support compounds etc. Designing catalyst with resistance to deactivation by the use of promoters is explored in this dissertation. The influence of promoters on the initiation pathways of CO hydrogenation is also explored as a first step towards determining the selectivity of promoted catalyst. The influence of Pt promoter on O removal from the surface of Co catalyst showed that Pt promoter reduced the activation barrier for the removal of O on both flat and stepped Co surfaces. An approximate kinetic model was developed and a volcano plot was established. The turn-over frequency (TOF) calculated based on the activation barriers showed that Pt promoted Co surface had a higher rate than unpromoted Co surface. The effect of Pt and Ru promoters on various pathways of C deposition on Co catalyst was studied to gain a mechanistic understanding. The promoters did not affect the subsurface C formation but they increased the barriers for C-C and C-C-C formation and also decreased the barriers for C-H formation. The promoters also influence the stabilities of C compounds on the Co surface suggesting that Pt and Ru promoters would decrease C deposition on Co catalysts. The effect of Pt promoter on unassisted and H-assisted CO activation pathways on Co catalyst was studied. Pt promoted Co surface followed H-assisted CO activation. Pt promoter decreased the activation barriers for CO activation pathways on Co catalyst thereby increasing the activity of Co catalyst. CO oxidation is a process used to prevent poisoning of fuel cell catalysts and reduce pollution of the atmosphere through exhaust gases containing CO. Expensive catalysts like Pt are widely used for CO oxidation which significantly increases the cost of the process and hence it is necessary to search for alternative lower cost catalysts. Understanding the mechanism of a reaction is the first step towards designing better and efficient catalyst. DFT is helpful in determining the basic mechanism and intermediates of reactions. The mechanism of CO oxidation on CoO catalyst was explored. Four possible mechanisms for CO oxidation on CoO catalyst were studied to determine the most likely mechanism. The mechanism was found to be a two-step process with activation barrier for formation of CO2 larger than the barrier for formation of the intermediate species.

The synthesis and characterization of Pt core/ Cu shell (Pt@Cu) dendrimer-encapsulated nanoparticles (DENs) having full and partial Cu shells deposited via electrochemical underpotential deposition (UPD) is described. Pt DENs containing averages of 55, 147, and 225 Pt atoms immobilized on glassy carbon electrodes served as the substrate for the UPD of a Cu monolayer. This results in formation of Pt@Cu DENs. Evidence for this conclusion is based on results from the analysis of cyclic voltammograms (CVs) for the UPD and stripping of Cu on Pt DENs, and from experiments showing that the Pt core DENs catalyze the hydrogen evolution reaction before Cu UPD, but that after Cu UPD this reaction is inhibited. Results obtained by in-situ electrochemical X-ray absorption spectroscopy (XAS) confirm the core@shell structure. Calculations from density functional theory (DFT) show that the first portion of the Cu shell deposits onto the (100) facets, while Cu deposits lastly onto the (111) facets. The DFT-calculated energies for Cu deposition on the individual facets are in good agreement with the peaks observed in the CVs of the Cu UPD on the Pt DENs. Finally, structural analysis of Pt DENs having just partial Cu shells by in-situ XAS is consistent with the DFT-calculated model, confirming that the Cu partial shell selectively decorates the (100) facets. These results are of considerable significance because site-selective Cu deposition has not previously been shown on nanoparticles as small as DENs. In summary, the application of UPD as a synthetic route and characterization tool for core@shell DENs having well defined structures is established. A study of the degradation mechanism and degradation products of Pd DENs is provided as well. These DENs consisted of an average of 147 atoms per dendrimer. Elemental analysis and UV-vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in air-saturated solutions, less oxidation in N₂-saturated solution, and no detectable oxidation when the DENs are in contact with H₂. Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd²⁺-complexing ligands such as chloride. For the air- and N₂-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy. Calculations from density functional theory (DFT) show that the first portion of the Cu shell deposits onto the (100) facets, while Cu deposits lastly onto the (111) facets. The DFT-calculated energies for Cu deposition on the individual facets are in good agreement with the peaks observed in the CVs of the Cu UPD on the Pt DENs. Finally, structural analysis of Pt DENs having just partial Cu shells by in-situ XAS is consistent with the DFT-calculated model, confirming that the Cu partial shell selectively decorates the (100) facets. These results are of considerable significance because site-selective Cu deposition has not previously been shown on nanoparticles as small as DENs. In summary, the application of UPD as a synthetic route and characterization tool for core@shell DENs having well defined structures is established. A study of the degradation mechanism and degradation products of Pd DENs is provided as well. These DENs consisted of an average of 147 atoms per dendrimer. Elemental analysis and UV-vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in air-saturated solutions, less oxidation in N2-saturated solution, and no detectable oxidation when the DENs are in contact with H2. Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd2+-complexing ligands such as chloride. For the air- and N2-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy. / text

This dissertation includes work done on graphene and related materials, examining their electronic and topological properties using first-principles methods. Ab-initio computational methods, like density functional theory (DFT), have become increasingly popular in condensed matter and material science. Motivated by the search for novel materials that would help us devise fast, low-power, post-CMOS transistors, we explore the properties of some of these promising materials. We begin by studying graphene and its interaction with dielectric oxides. Graphene has recently inspired a flurry of research activity due to its interesting electronic and mechanical properties. For the device community, graphene's high charge carrier mobility and continuous gap tunability can have immense use in novel transistors. In Chapter 3 we examine the properties of graphene placed on two oxides, namely quartz and alumina. We find that oxygen-terminated quartz is a useful oxide for the purpose of graphene based FETs. Inspired by a recent surge of interest in topological insulators, we then explore the topological properties of two-dimensional materials. We conduct a theoretical study to examine the relationship between crystal space group symmetry and the electric polarization of a two-dimensional crystal. We show that the presence of symmetry restricts the polarization values to a small number of distinct groups. There groups in turn are topologically inequivalent, making polarization a topological index. We also conduct density functional theory calculations to obtain actual polarization values of materials belonging to C3 symmetry and show that our results are consistent with our theoretical analysis. Finally we prove that any transformation from one class of polarization to another is a topological phase transition. In Chapter 5 we use density functional theory to examine the electronic properties of graphene intercalation compounds. Bilayer pseudospin field effect transistor (BiSFET) has been proposed as an interesting low-power, efficient post-CMOS switch. In order to implement this device we need bilayer graphene with reduced interlayer interaction. One way of achieving that is by inserting foreign molecules between the layers, a process which is called intercalation. In this chapter we examine the electronic properties of bilayer graphene intercalated with iodine monochloride and iodine monobromide molecules. We find that intercalation of graphene indeed makes it promising for the implementation of BiSFET, by reducing interlayer interaction. As an interesting side problem, we also use hybrid, more extensive approaches in DFT, to examine the electronic and optical properties of dilute nitrides. Dilute nitrides are highly promising and interesting materials for the purposes of optoelectronic applications. Together, we hope this work helps in elucidating the electronic properties of promising material systems as well as act as a guide for experimentalists. / text

Atomically two-dimensional (2-D) graphene, as well as the hexagonal boron nitride dielectric have been and are continuing to be widely investigated for the next generation nanoelectronic devices. More recently, other 2-D materials and electronic systems including the surface states of topological insulators (TIs) and monolayers of transition metal dichalcogenides (TMDs) have also attracted considerable interest. In this work I have focused on these latter two material systems on possible device applications. TIs are characterized by an insulating bulk band gap and metallic Dirac surface states which are spin-polarized. Here, the electronic structures of bulk and thin film TIs are studied using ab-initio density functional theory (DFT). Band inversion, an essential characteristic of TIs, is shown in the bulk band structures. Properties of TI surface bands in thin film such as the critical film thickness to induce a gap, the thickness dependent gap size, and the localization length of surface states are reported. Effects of crystalline dielectric materials on TI surface states are also addressed by ab-initio calculations. I discuss the sensitivity of Dirac point degeneracy and linear band dispersion of TI with respect to different dielectric surface terminations as well as different relative atom positions of the dielectric and TI. Additionally, this work presents research on exciton condensation in TI using a tight-binding model combined with self-consistent non-local Hartree-Fock mean-field theory. Possibility of exciton condensation in the TI Bi₂Se₃ thin film is assessed. Non-equilibrium Green's function (NEGF) simulations with the atomistic tight-binding (TB) Hamiltonian are carried out to explore the performance of metal-oxide-semiconductor field-effect-transistor (MOSFET) and tunnel field-effect-transistor (TFET) based on the Bi₂Se₃ TI thin film. How the high dielectric constant of Bi₂Se₃ affects the performance of MOSFET and TFET is presented. Bulk TMDs such as MoS₂, WS₂ and others are the van der Waals-bonded layered material, much like graphite, except monolayer (and Bulk) TMDs have a large band gap in-contrast to graphene (and graphite). Here, the performance of nanoscale monolayer MoS₂ n-channel MOSFETs are examined through NEGF simulations using an atomistic TB Hamiltonian. N- and p-channel MOSFETs of various monolayer TMDs are also compared by the same approach. I correlate the performance differences with the band structure differences. Finally, ab-initio calculations of adatom doping effects on the monolayer MoS₂ is shown. I discuss the most stable atomic configurations, the bonding type and the amount of charge transfer from adatom to the monolayer MoS₂. / text

Transition metal alloys are an important class of materials in heterogeneous catalysis due in no small part to the often greatly enhanced activity and selectivity they exhibit compared to their monometallic constituents. A host of experimental and theoretical studies have demonstrated that, in many cases, these synergistic effects can be attributed to atomic-scale features of the catalyst surface. Realizing the goal of designing -- rather than serendipitously discovering -- new alloy catalysts thus depends on our ability to predict their atomic configuration under technologically relevant conditions. This dissertation presents original research into the development and use of computational tools to accomplish this objective. These tools are all based on a similar strategy: For each of the alloy systems examined, cluster expansion (CE) Hamiltonians were constructed from the results of density functional theory (DFT) calculations, and then used in Metropolis Monte Carlo (MC) simulations to predict properties of interest. Following a detailed description of the DFT+CE+MC simulation scheme, results for the AuPd/Pd(111) and AuPt/Pt(111) surface alloys are presented. These two systems exhibit considerably different trends in their atomic arrangement, which are explicable in terms of their interatomic interactions. In AuPd, a preference for heteronuclear, Au-Pd interactions results in the preferential formation of Pd monomers and other small ensembles, while in AuPt, a preference for homonuclear interactions results in the opposite. AuPd/Pd(100) and AuPt/Pt(100) were similarly examined, revealing not only the effects of the same heteronuclear/homonuclear preferences in this facet, but also a propensity for the formation of second nearest-neighbor pairs of Pd monomers, in close agreement with experiment. Subsequent simulations of the AuPd/Pd(100) surface suggest the application of biaxial compressive strain as a means increasing the population of this catalytically important ensemble of atoms. A method to incorporate the effects of subsurface atomic configuration is also presented, using AuPd as an example. This method represents several improvements over others previously reported in the literature, especially in terms of its simplicity. Finally, we introduce the dimensionless scaled pair interaction, whereby the finite-temperature atomic configuration of any bimetallic surface alloy may be predicted from a small number of relatively inexpensive calculations. / text